S. L. Bendell in his U.S. Pat. App. Ser. No. 532,958 filed Sept. 16, 1983, entitled "TELEVISION CAMERA WITH SOLID STATE IMAGERS COOLED BY A THERMAL SERVO" and assigned to RCA Corporation, describes the cooling of solid-state imagers to prescribed temperatures. These temperatures are low enough that the dark currents on all solid-state imagers within specification are reduced sufficiently that noise attributable to dark current variations is kept below acceptable limit in television pictures produced from the output signals of those imagers. In the back-illuminated thinned-substrate CCD imagers presently manufactured by RCA Corporation, cooling the solid-state imager from normal operating temperature down to 10.degree. C. will reduce noise attributable to dark current variations fourfold, to levels comparable to noise from other sources. This cooling need is much more modest than the need for cooling to liquid nitrogen temperatures, as required with visible light sensing CCD imagers used in astronomical observations and similar applications, or with infrared-sensing CCD imagers. Cooling can be done satisfactorily with thermoelectric coolers bonded to the CCD imagers. The particular of bonding a thermoelectic cooler to a CCD imager to avoid condensation on the imager surface exposed to illumination are described by P. D. Southgate in his U.S. Pat. App. Ser. No. 532,957 filed Sept. 16, 1983, entitled "CAMERA WITH REDUCED-CONDENSATION COOLED CCD IMAGER", and assigned to RCA Corporation.
The sensing of the temperature of the solid-state imager was carried out in the previous apparatus using the potential drop across a forward-biased semiconductor junction located on the same semiconductor die as the imager. Complications in the processing of the semiconductor die are introduced by the need to avoid injection of charge carriers by the forward-biased junction into the imager, and it is impractical to locate the forward-biased junction in the central portions of the imager where one would prefer to sense temperature. Also, this method senses temperature at a point on the chip, rather than over an entire area where the dark currents of concern are generated. Also, there is a problem of providing an extra pin on the imager package to access the sensor output signal.
The present inventor in his U.S. Pat. No. 4,496,982 issued Jan. 29, 1985, entitled "COMPENSATION AGAINST FIELD SHADING IN VIDEO FROM FIELD-TRANSFER CCD IMAGERS" and assigned to RCA Corporation, and D. D. Crawshaw in his U.S. Pat. No. 4,498,105 issued May 2, 1985, entitled "FIELD TRANSFER CCD IMAGERS WITH REFERENCE BLACK-LEVEL GENERATION CAPABILITY", and assigned to RCA Corporation describe various ways of sensing accumulated dark current on a semiconductor die into which a CCD imager is integrated. Accumulated dark current can be extracted from the imager in several ways through pins already existent on the imager package for other purposes. Accumulated dark current increases in a known way with temperature, doubling for every 10.degree. C. or so increase in temperature in a silicon device. The accumulated dark current can be used to meter temperature change, rather than using a forward-biased semiconductor junctions, thereby to improve the thermal servomechanism described in S. L. Bendell's U.S. Pat. App. Ser. No. 532,958. A thermal servomechanism so improved conserves the power needed for cooling, reducing the cooling when the imager is operated in inherently cooler surroundings or is exposed to less radiant energy.
But, the ultimate goal of cooling the solidstate imager is to limit the level of dark current variations. The level of the dark current variations from picture element to picture element (i.e. from pixel to pixel) in a well-made modern solid-state imager is substantially linearly related to the average dark current level, over extensive time and over an extensive number of pixels, since these variations are essentially Johnson noise. One can measure average dark current level by sensing accumulated dark current along the lines described or suggested in U.S. Pat. No. 4,496,982, then use the measurement to control the amount of cooling applied to the solid-state imager. This allows regulating directly for constant average dark current level, and thus indirectly for constant noise owing to variations in dark current level, rather than regulating for constant imager temperature.
Indeed, regulating for constant average dark current level is a preferable way to control the temperature of the solid-state imager. One finds variations by a factor of two to three in dark current levels from one solid-state imager to another--owing, for example, to variation in the crystalline structure of the semiconductor substrate and to variations in the processing to emplace an imager on the substrate. Regulating for a prescribed dark current level, rather than a prescribed imager temperature, means that imagers with inherently less dark current and less attendant variation in dark current level automatically will be provided with less cooling. Reducing cooling reduces power consumption in the camera, and it prolongs battery life in a battery-powered camera. When one replaces the imager in a camera, there is no need to adjust temperature to reduce dark current or to minimize power consumption for satisfactorily low dark current noise. Further, there is uniformity from camera to camera across a product line, which lessens the likelihood of customer returns for "unsatisfactory" dark current performance as compared to another camera of the same type. In broadcasting service, switches from camera to camera are less likely to be noticeable.
The measurement of dark current generated in a charge coupled device array of the size of an imager field storage register can provide temperature measurement with resolution to millidegrees Celsius. Each dark-current charge accumulating location in a CCD area array of a size similar to a location in a CCD imager field storage (B) register over a 1/60 second period can accumulate on average 2,000 electrons at room temperature. This 2000 electron average has about 40 electrons r-m-s noise variation from location to location. When the respective 2,000 electrons charge contributions from an array of locations n in number are merged, these contributions add arithmetically to give 2000n electrons. A full-well charge of 200,000 electrons can be provided by 100 dark-current charge accumulating locations. The 40 electrons r-m-s noise from each location adds vectorially to 40(n).sup.(1/2) electrons r-m-s or 400 electrons r-m-s over 100 locations. The number of accumulated electrons doubles each eight degrees Celsius. In the eight degrees up to room temperature half the full-well charge of 200,000 electrons is accumulated with this 40 electrons r-m-s noise, so temperature resolution is 8.degree. C./(100,000/400) or 32 millidegrees Celsius, which is good for the short sampling period and improves as temperature increases. So, then, a moderate-size CCD area array of 100 locations or so can be placed on any semiconductor die the temperature of which is to be regulated, and used to sense die temperature for the thermal regulation loop.